1. Field of the Invention
The present invention relates to a coal-water mixture using an upgraded low-rank coal obtained by upgrading a low-rank coal of high water content, as well as to a process for producing said mixture. The coal-water mixture can be used as a fuel as it is.
2. Description of the Prior Art
Many of low-rank coals having a carbon content of 60-75% by weight on a dry ash-free basis, have a water (inherent water) content specified by Japanese Industrial Standard (JIS) M 8812, (1984), pages 1-25, of as high as 20-30%, and some of them show a water (equilibrium moisture) content at room temperature and 98% relative humidity, of as high as 60-70% depending upon the kind of coal. Transportation of such high-water-content coals as they are is uneconomical and the efficiency in their use as a fuel or in other applications is low. In order to reduce the water content in coal, it is considered to dry coal by, for example, hot-air or natural draft; however, the dried coal reabsorbs water during storage or transportation and further there is a fear that during the process of water reabsorption the coal undergoes pulverization or invites spontaneous combustion. Because of these problems, low-rank coals find limited applications, and most of them remain unutilized although their reserves are very large.
In order to seek effective utilizations of low-rank coals of high water content, there were proposed various methods for the upgrading of low-rank coal by lowering the equilibrium moisture content of the coal. As such methods, there are known, for example, (1) a Fleissner method which comprises lowering the water content in coal by using a saturated steam of a temperature of 473.degree.-573.degree. K. and a pressure of 1.5-8.5 MPa [e.g., T. G. Rozgonyi and I. Z. Szigeti, International J. Mining and Eng., 2, (1984), 157-169] and (2) a hydrothermal method which comprises upgrading a coal-water mixture in a pressurized water of a temperature of 473.degree.-603.degree. K. and a pressure of 1.5-17 MPa [e.g., T. A. Potas, R. E. Sears, D. J. Maas, G. G. Baker and W. G. Willson, Chem. Eng. Commun. Vol. 44, (1986), pp. 135-151]. These methods are effective for the upgrading and consequent water reduction of low-rank coal; however, equilibrium moisture content is reduced to about 11-20% at best. A higher treating temperature and a longer treating time tend to give a higher level of upgrading, but a lower temperature and a shorter time are desirable when the treatment is conducted industrialy. Further, the treating temperature is limited by the pressure of the saturated steam employed. Owing to these restrictions, equilibrium moisture content is reduced to about 11% at best in the above methods. Equilibrium moisture content reduction of such a degree is insufficient for use of low-rank coals in wider applications.
On the other hand, there is known a method for coal treatment using water and carbon monoxide [the technological field of this method is different from that of the present invention process for coal pretreatment (coal upgrading)]. As such a method, there is known a technique which comprises heating coal and the oil derived from coal in the presence of a very small amount of water and carbon monoxide at a temperature higher than the critical temperature (647.3 K) of water (i.e. in a state that no liquid water exists) to give rise to a water gas reaction between water and carbon monoxide (CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2) and allowing the nascent hydrogen to act on the coal in the presence of a particular catalyst to give rise to coal liquefaction [Yokoyama et al., Journal of the Fuel Society of Japan, 57, (1977), 182-189]. This technique aims at coal decomposition and liquefaction and is therefore essentially different from and not applicable to a technique for reduction of water occluded in coal.
Thus, a method for upgrading low-rank coal to a reduced water content has heretofore been sought for wider application of such coal.
Various proposals were also made to convert a coal to a liquid fuel which is easy to transport, which can be used as a fuel as it is, and which is easy to handle for combustion. One of such proposals relates to a coal-water mixed slurry (hereinafter referred to as CWM). Many researches were already made on the production of CWM, and a number of factors governing the properties of CWM were found. The most important factor governing the CWM properties is the amount of water occluded in coal. That is, for production of a CWM of high coal concentration, it is desirable that the content of water occluded in the fine pores of coal particles (said water reduces the calorie of CWM but makes no contribution to the improvement of CWM fluidity) be as small as possible. Since the amount of water occluded in the fine pores of coal particles is thought to be approximately proportional to the amount of equilibrium moisture of the coal particles present in an atmosphere of room temperature and a 98% or more relative humidity, said amount of equilibrium water can be used as a yardstick for the producibility of CWM.
Viewed from the equilibrium water content, low-rank coals as mentioned above, each have a large amount of occluded water. Because of the large amount of occluded water, low-rank coals have been difficult to convert to a CWM of high calorie. In order to produce a good CWM, it is necessary to pay attention not only to the amount of water occluded in coal used as a material but also to the storage- or transportation-related properties (e.g. fludity, stability) of CWM produced.
The properties of CWM (e.g. coal content, viscosity, stability) are greatly influenced by the amount of water present in the CWM together with coal particles. A relatively small water amount of 2-3% based on coal has a detrimental effect on CWM properties, in many cases.
Hence, in order to obtain a good CWM, there must be found a solution to technical tasks which seem contradictory with each other, i.e. (1) a small amount of water occluded in coal particles and (2) the presence of water between coal particles.